In telecommunications networks, especially public land mobile networks or telecommunications networks for mobile communication, there is a need to provide services of the telecommunications network according to different kinds of accesses or attachments of user equipments to the telecommunications network (or different radio access technologies, RAT), such as, e.g. 2G access, 3G access or 4G access.
The term 2G access relates to second generation mobile telecommunications networks and corresponding user equipments, typically denoted by the GSM standard (Global System for Mobile communication). The term 3G access relates to third generation mobile telecommunications network and corresponding user equipments, typically denoted by the UMTS standard (Universal Mobile Telecommunication System). The term 4G access relates to fourth generation mobile telecommunications network and corresponding user equipments, typically denoted by the LTE standard (Long Term Evolution).
According to older radio access technologies (i.e. older kinds of access or attachment of user equipments to the telecommunications network), especially 2G and 3G standards, the network is required to provide the possibility (to user equipments) of a CS access (circuit switched access) or CS attachment (circuit switched attachment) to the telecommunications network, whereas according to more recent radio access technologies, especially the 4G standard, the network is required to provide a PS access (packet switched access) or PS attachment (packet switched attachment).
The specific costs (i.e. costs related to serving a certain number of subscribers or transmitting a certain volume of data) associated with maintaining and operating legacy components of telecommunications networks, especially components providing CS (circuit switched) attachment of user equipments is typically growing relative to the specific costs associated with components providing PS (packet switched) attachments of user equipments. As a consequence, the relative share of PS capabilities of typical telecommunications networks, especially according to or implementing the IP Multimedia Subsystem (IMS), typically grows.
The IP Multimedia Subsystem (IMS) is a framework for delivering multimedia services based on IP (Internet Protocol) connections to mobile user equipments. Typically, IMS uses IETF (Internet Engineering Task Force) protocols, e.g., SIP (Session Initiation Protocol).
Despite the rise of PS capabilities of typical telecommunications networks (relative to CS capabilities), CS connectivity needs to be assured by the telecommunications network to maintain services for those user equipments or subscribers that are not able to use the PS capabilities. It is desirable to apply an IMS-oriented handling of telecommunications services also in the case of CS connectivity. In case of such an IMS-oriented handling of telecommunications services, an IMS identity and/or one IMS profile (or a plurality of IMS profiles) is required for each subscriber. Typically both an IMS identity and one or a plurality of IMS profiles (associated to a user equipment or associated to a subscriber) are required for IMS handling. An example of the IMS identity is “IP Multimedia Private Identity” (IMPI), which is a unique permanently allocated global identity assigned by the home network operator, and is typically used for Registration, Authorization, Administration, and Accounting purposes. Normally, every IMS user shall have one IMPI.
However, in a roaming scenario, it cannot be assumed or guaranteed for all roaming partners of a given telecommunications network that such an IMS identity and/or an IMS profile (or a plurality of IMS profiles) exist for a given user equipment or subscriber roaming in the telecommunications network.